i think oscars are great but im sure they could nip at the rays much like alot of fish though.
along time ago i had an oscar with my rays and i thought it was big enough but there is a pic on here somewhere where the oscar is pinned upsided down against the glass by one of the rays.
the rays will allways try to chew anything they can find in my experience. so you need to make sure whatever it is is able to look after themselves.
i think a decent oscar would be alrirght but i recently had a problem with a smaller geo nipping at a rays tail which left the ray with a short tail.
as with everything you just have to keep an eye on them. anything with preditors is a risk, you just have to be carefull..
good luck .